When threading a sewing machine the needle thread is usually directed from a source of supply such as a spool or the like through thread guides, a thread tensioning device, a thread take-up mechanism and finally to the needle. The tension device itself generally includes a plurality of thread friction washers or discs between which the thread is placed during operation of the sewing machine. The thread friction discs may be spring biased towards one another with the spring pressure being adjustable so that the tension on the thread may be varied. The thread upon leaving the thread supply may pass through a stationary thread guide, between the tension discs and through a check-spring which may be carried by the tension device itself. Such an arrangement is shown in U.S. Pat. No. 3,667,414 granted June 6, 1972. As also disclosed in the aforementioned patent, the tension device may be operatively connected with the presser bar mechanism so that when the presser bar is lifted the presser foot is raised above the work and the tension will be released in the tension device and when the presser foot is lowered by the presser bar mechanism the tension will be restored in the tension device.
When the tension is released in the tension device so that the discs are relatively spaced apart and the thread is thereby free to move with respect to the discs, a problem arises that the thread may leave its operative position with respect to the tension discs and when tension is restored in the tension device the thread will not be properly disposed within the tension device which may result in an undesirable sewing operation. This problem may occur more frequently in a situation wherein the presser bar is raised along with the presser foot and in combination with the tension device the tension is released in the tension device thereby relieving any tension or pressure on the thread. Since the thread is free to move at this time and since the action of a thread having previously been under tension is somewhat unpredictable, the thread may move to a position where it is not properly disposed within the tension device. Unless the operator is aware of such condition, when the presser mechanism is lowered, the thread may not be properly disposed within the tension device thus giving rise to a situation wherein the tension device has no effect on the thread and a possible defect in the resulting stitching.
It is a purpose of the present invention to overcome the problem of possible unwanted unthreading of the thread from the tension device during operation of the presser bar mechanism.